Decadal scale phytoplankton species miniaturization in subtropical coastal waters.

Miniaturization, i.e. reduction in body size, happens in different organisms as an adaptation strategy under environmental stress such as warming. However, whether phytoplankton miniaturization occurs in coastal waters remains understudied due to complex environmental factors and strong spatiotemporal variability. Here, we comprehensively investigated the long-term changes in phytoplankton body size over 20 years in the coastal waters of Hong Kong through monthly sampling at 25 stations across the region. We employed a framework distinguishing two drivers of community miniaturization: (i) intraspecific size reduction (species miniaturization) and (ii) shifts in community composition toward a higher proportion of small species. At the species level, miniaturization was widespread, more in diatoms than dinoflagellates, primarily driven by temperature, supporting the temperature-size relation. In contrast, community-level miniaturization was negligible across most stations (except in a semi-closed bay), which was attributed to the decreased proportion of small species. This could be explained by the declined phosphate concentration which not only directly reduced the proportion of small species but also diminished the temperature sensitivity of phytoplankton community. Our findings provide multiscale insights into coastal phytoplankton miniaturization, with critical implications for food web dynamics and the biological carbon pump. Moreover, we highlight that anthropogenic nutrient reduction may significantly mitigate community-level phytoplankton miniaturization, though localized effects in semi-enclosed systems warrant further investigation.